1. Field of the Invention
This invention relates to a method of reading out a radiation image wherein final read-out is carried out for detecting image signals for use in reproduction of a visible image from a recording medium such as a stimulable phosphor sheet carrying a radiation image recorded thereon, and an apparatus for carrying out the method. This invention particularly relates to a method of reading out a radiation image so that the density of an object region concerned in a visible image reproduced based on the read-out image signals is prevented from differing between images of the object recorded in different image recording directions, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon. Then, final read-out is carried out by exposing the stimulable phosphor sheet carrying the radiation image stored thereon to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light to obtain electric image signals for use in reproduction of a visible image. An image processing is carried out on the final read-out image signals obtained by the final read-out, and the radiation image of the object is reproduced as a visible image by use of the processed image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, the range of the level of the radiation energy stored on the stimulable phosphor sheet is caused to fluctuate among radiation images by changes in the object, the image recording portion thereof, radiation dose, or the like.
However, in the aforesaid radiation image recording and reproducing system, the recorded image information, particularly the range of the level of the radiation energy or the like, of each radiation image stored on the stimulable phosphor sheet may be ascertained in advance, and the final read-out may be carried out by use of read-out conditions such as a read-out gain and a scale factor adjusted to appropriate values in accordance with the recorded image information of each radiation image. In this case, for each radiation image, it becomes possible to obtain a visible image free from adverse effects of the fluctuation in the range of the level of the radiation energy stored on the stimulable phosphor sheet and suitable for viewing, particularly for diagnostic purposes, for example, a visible image wherein the necessary object image information is always expressed within the correct density range suitable for viewing, particularly for diagnostic purposes.
Also, in the aforesaid radiation image recording and reproducing system, the image processing of the final read-out image signals obtained by the final read-out is carried out by use of image processing conditions such as gradation processing conditions adjusted for each radiation image based on the image recording portion of the object such as the head, the chest or the abdomen, and/or the image recording method such as plain image recording or contrasted image recording so that a visible image suitable for viewing, particularly for diagnostic purposes, can be obtained. However, for example, in the case where the final read-out is carried out without using the read-out conditions adjusted to appropriate values in accordance with the recorded image information of each radiation image, the image processing conditions should preferably be adjusted by considering the recorded image information of each radiation image, which has been ascertained in advance, besides the image recording portion of the object and/or the image recording method. In this manner, it becomes possible to obtain a visible image suitable for viewing, particularly for diagnostic purposes, wherein the necessary object image information is expressed within the correct density range.
Ascertaining of the image information recorded on the stimulable phosphor sheet prior to the final read-out and the image processing may be carried out by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, preliminary read-out for ascertaining the recorded image information of a radiation image stored on the stimulable phosphor sheet is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in the final read-out for detecting the image signals for use in reproduction of a visible image for viewing, particularly for diagnostic purposes, and thereafter the final read-out is carried out. The read-out conditions for the final read-out and/or the image processing conditions are adjusted based on the recorded image information ascertained by the preliminary read-out, i.e. the preliminary read-out image signals obtained by the preliminary read-out.
As mentioned above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read out. Specifically, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out.
In the case where only the image processing conditions are to be adjusted based on the recorded image information, it is only necessary that the recorded image information be ascertained prior to the image processing. In this case, since the final read-out image signals representing the recorded image information have already been detected, the image processing conditions can be adjusted based on the final read-out image signals, and the preliminary read-out as mentioned above need not necessarily be carried out.
However, in the case where the radiation image read-out conditions for the final read-out and/or the image processing conditions are adjusted as mentioned above, the density of a region concerned in the object often differs between reproduced visible images when radiation images of the same object are recorded in different image recording directions. This problem will hereinbelow be described in detail. FIG. 2A shows the case where an image of the chest is recorded from the front of the object for diagnosis of the thoracic vertebrae, and FIG. 2B shows the case where an image of the chest is recorded from the side of the object. In the case of the front image recording as shown in FIG. 2A, since the thoracic vertebrae K which are in the image-recording region overlap the mediastinum through which radiation cannot readily pass, the amount of the radiation stored at the thoracic vertebra image portion on a stimulable phosphor sheet 103 is small, and only a small amount of light is emitted by the thoracic vertebra image portion on the stimulable phosphor sheet 103 as shown by the histogram of the read-out image signals in FIG. 3A when the sheet 103 is exposed to stimulating rays. On the other hand, in the case of the side image recording as shown in FIG. 2B, since the thoracic vertebrae K overlap the lungs through which radiation can readily pass, the amount of radiation stored at the thoracic vertebra image portion on a stimulable phosphor sheet 103 is large, and a large amount of light is emitted by the thoracic vertebra image portion on the stimulable phosphor sheet 103 as shown by the histogram of the read-out image signals in FIG. 3B when the sheet 103 is exposed to stimulating rays. Since the maximum value Smax and the minimum value Smin of the read-out image signals detected from the stimulable phosphor sheet do not much differ between the front image recording and the side image recording, the read-out conditions for the final read-out and/or the image processing conditions adjusted by a method of adjustment based on the maximum value Smax and the minimum value Smin of the read-out image signals as one of the conventional methods of adjustment based on the recorded image information become approximately equal between the front image recording and the side image recording. Therefore, when the image read-out is carried out and a visible image is reproduced by use of the read-out conditions and/or the image processing conditions adjusted in this manner, the density of the thoracic vertebra image portion becomes comparatively low in the reproduced visible image obtained in the case of the front image recording, and becomes comparatively high in the case of the side image recording.
Besides the aforesaid image recording portion of the object and the aforesaid image recording directions, the problems as mentioned above generally arise between the case where an object portion exhibiting a particularly high or low radiation absorptivity overlaps an image-recording region of an object in the course of image recording and the case where such an object portion does not overlap the object region concerned, depending on the image recording direction of the object.
If the image density of the object region concerned differs between radiation images recorded in different image recording directions of the object, it is not always possible to accurately make diagnosis by the utilization of the radiation images.
The problems as mentioned above may arise also when a radiation image recorded on a recording medium other than the stimulable phosphor sheet is read out to reproduce a visible image.